This invention relates to a method and apparatus for producing high purity titanium crystal and titanium and high purity titanium crystal and titanium so produced. The invention involves producing titanium sponge and performing titanium fused salt electrolysis in situ in the same container in which titanium sponge is produced to produce high purity titanium crystals and, where especially low oxygen content is desired, to treat the high purity titanium crystal as produced with iodine described herein.
Currently, high purity Ti is used extensively in the fabrication of microprocessors and similar components for the microelectronics industry. Constantly evolving microprocessor capabilities (increasing integration, decreasing circuit element dimensions) have driven producers to meet the demands of chip manufacturers for increasingly pure titanium.
High purity Ti targets are used in sputtering applications to produce thin films on microprocessors, integrated circuits, DRAMs, flat panel displays, etc. Purity requirements range from 99.99% (4N) to 99.99999% (7N), depending on the application. For improved end usage properties, it is of interest to decrease impurity elements including, but not limited to, alkali metals such as Na and K, heavy metals such as Fe, Ni and Cr, radio active elements such as U and Th, and gases, especially oxygen. These elements have undesirable affects on properties; for example, Fe contamination degrades film patterning and circuit element registry, oxygen affects the resistivity of the deposited film; Na and K migrate from deposited films into active transistor elements thus degrading performance, and U and Th are both alpha-emitters which can cause adjacent solid state switches to change state. So called xe2x80x9c6Nxe2x80x9d purity Ti, where total impurities (excluding gases) are less than 1 ppm, and with low oxygen, e.g.  less than 100 ppm, is presently desired in the industry for many of the above applications. Lowering the amounts of impurity elements can improve the performance of the sputtering process and increase the reliability and speed of the microprocessor or memory device in which the high purity titanium is used.
Common processes for producing high purity titanium include: (1) producing Ti sponge is by reacting TiCl4 with a reducing agent such as Mg or Na in a vessel designed for this purpose, (2) vacuum distilling Ti sponge in a vacuum distilling apparatus to remove residual salt or otherwise treat sponge to remove residual salt, (3) electrolyze Ti sponge in an electrolytic cell by fused salt electrolysis, and (4) melting Ti using an electron-beam furnace or similar high-vacuum melting process. Titanium sputtering targets may be produced from ingot produced by a combination of these processes.
In accordance with the invention there is provided a method and apparatus for producing high purity titanium and high purity titanium so produced.
The method comprises reacting titanium tetrachloride with a reducing agent comprising an alkali and/or alkaline earth metal in a container to produce titanium sponge and molten salt, chlorinating the salt in the same container, electrolyzing the titanium sponge in situ in the same container to produce titanium crystal, recovering titanium crystal from the container and removing salt from the titanium crystal. Advantageously, the container is provided with a reactant-contacting surface comprising a metal electrochemically more noble than titanium according the chloride electromotive series. Preferably, the reducing agent comprises sodium, magnesium, potassium, calcium and/or lithium or alloys of each together, and the reactant-contacting surface preferably comprises molybdenum, nickel or molybdenum/nickel alloy.
A suitable apparatus in accordance with the invention comprises a container capable of of being maintained xe2x80x9cair tightxe2x80x9d provided with means for feeding raw material, a cathode and an anode and a reactant-contacting surface comprising a metal more electrochemically noble than titanium according to the chloride electromotive series. The apparatus is capable of accommodating both titanium sponge production and fused salt electrolysis and thus is capable of facilitating both reduction and electrolysis without removal of the reduction reaction products from the container prior to electrolysis. Salt produced during the reduction of titanium tetrachloride is capable of serving as the salt bath for the electrolysis process, after being chlorinated.
In another embodiment the high purity titanium crystal produced as described above is further reacted with iodine to produce TiI2 and TiI4 and thermally decomposed in a container at elevated temperature under vacuum to result in high purity titanium with an oxygen content less than about 50 ppm, and even less than about 30 ppm. Advantageously, the reaction temperature used is in the range of about 500xc2x0 C. to 800xc2x0 C., preferably about 750xc2x0 C. to 775xc2x0 C., the decomposition temperature is about 1150xc2x0 C. to 1450xc2x0 C., preferably about 1300xc2x0 C. to 1400xc2x0 C., and the pressure up to about 1000 microns, preferably about 100 to 500 microns.
This method comprises producing high purity titanium crystal as described, treating the titanium crystal with iodine to form titanium iodide and decomposing the iodide at elevated temperature and under reduced pressure to produce titanium crystal bar containing less than about 50 ppm oxygen.
The process of the invention is capable of producing high purity titanium sponge containing less than 1 ppm, by weight, total of Be, Mn, Zr, Al, and V and less than 1 ppm, by weight, total of Yb, Zn, Cr, Cd, B, and Sn. It is also possible to produce titanium sponge having less than 1 ppm, by weight, total of all the foregoing, that is, Be, Mn, V, Zn, Cr., Al, Yb, Cd, Zr, B and Sn.
High purity titanium crystal in accordance with the invention comprises titanium with less than 1 ppm, by weight, total of sodium, potassium, aluminum, iron, chromium and nickel, less than 1 ppb, by weight, total of uranium and thorium, and less than 10 ppb, by weight, total of sodium and potassium, excluding gases and mechanically entrapped salt.
It should be noted that the descriptions of titanium compositions herein expressly excludes mechanically entrapped salt, whether so stated or not. Thus, for example, expressions of titanium purity and references to amounts of impurities exclude any consideration of amount of salt that may be mechanically entrapped within the titanium. The term xe2x80x9cmechanically entrapped saltxe2x80x9d refers to, without limitation, salt, e.g. NaCl, which may be physically trapped in voids, cavities, crevices and/or pockets in titanium as crystal intergrowth occurs and is neither dissolved in or alloyed with the metal to any significant extent.
The method(s) of chemical analysis used to derive the chemical descriptions set forth herein are the methods known as glow discharge mass spectroscopy (GDMS) and furnace atomic absorption (FAA) for metallic elements and LECO gas analyzers for gas analysis, using calibration samples traceable to National Institute of Standards and Technology (NIST).